The present invention relates to a glow plug used for preheating a sub combustion chamber or a combustion chamber of a diesel engine and, more particularly, to a glow plug for a diesel engine, which is of a fast heat type and has improved heating characteristics and self-temperature saturability to obtain a long after glow time.
Generally, since a diesel engine has a poor startability at a low temperature, a glow plug is installed in a sub combustion chamber or in a combustion chamber, and current flows through it to generate heat. Thus, the glow plug increases an intake air temperature or is used as an ignition source, thereby improving startability of the engine. A typical conventional glow plug of this type is of sheath type obtained by charging a heat-resistant insulating powder in a metal sheath and burying a coil heating wire made of iron, chromium, nickel, or the like therein. However, as shown in Japanese Patent Prepublication (Kokai) No. 57-41523 and the like, a ceramic type glow plug using a rod heater obtained by burying a heating wire made of, e.g., tungsten, in an insulating ceramic material such as silicon nitride is also known. As compared to the sheath type glow plug utilizing indirect heating through a heat-resistant insulating powder and a sheath, the latter, i.e., the ceramic heater type glow plug is superior in terms of heat transfer efficiency and heating characteristics, can be heated to red hot in a short time during heating to greatly improve temperature rise characteristics, can achieve performance as a fast heat type, and therefore is widely used in recent years.
However, the conventional ceramic heater type glow plug described above has a structure obtained by burying a heating wire made of a metal (tungsten) inside an insulating ceramic material such as silicon nitride. Since coefficients of thermal expansion of these two materials are different from each other, a problem, such as cracking, may be posed especially during heating due to rapid increase in temperature and repeated operations, resulting in poor durability, poor reliability such as heat resistance, and high cost of the ceramic heater.
In order to eliminate the above problems, Japanese Patent Prepublication (Kokai) Nos. 60-9085 and 60-14784 propose ceramic heater structures in which a heating wire is formed by a conductive ceramic material having substantially the same coefficient of thermal expansion as that of the insulating ceramic material. However, neither of them can be used as a glow plug in practice because of their structural and functional problems.
For example, the former is obtained by burying a conductive ceramic material serving as a heating body into an insulating ceramic material. Therefore, although it is superior to the sheath type in terms of thermal conductivity, it cannot serve as a fast heat type glow plug because of indirect heating, and fabrication thereof is cumbersome and complex. On the other hand, the latter can function as a fast heat type because a heating body is exposed at the side of the heater surface. However, since the heating body is simply formed by a stacking structure of members to obtain a U-shape and both ends thereof are merely connected to the rear end of the heater, the electrode structure becomes complex, resulting in high cost.
Furthermore, in the conventional ceramic heater type glow plugs, the heating section is formed by a junction structure of a conductive ceramic material and an insulating ceramic material. Therefore, although their coefficients of thermal expansion are substantially the same, their reliability is poor as a ceramic heater in a glow plug whose heating temperature is 1,100.degree. C. at maximum.
In addition, in the glow plug of this type, a demand for adopting a so-called after glow method is great in the market in recent years. In this method, durability against high temperatures of operating conditions is high due to improvements in startability of a diesel engine and adoption of turbo system. The glow plug is kept energized for a predetermined time interval after the engine is started to smoothly perform combustion inside the engine, so that exhaust and noise countermeasures are obtained. Furthermore, this after glow time must be as long as possible (e.g., 10 minutes). In order to achieve such a long after glow time, energization power must be self-controlled to greatly improve heating characteristics and prevent over heat at the heater, and a self temperature saturation function must be provided to maintain a saturation temperature below a proper temperature. Thus, a glow plug having a ceramic heater which has fast heat and self temperature saturation characteristics, which is superior in reliability such as heat resistance, and which can be manufactured at low cost is has been desired.